1. Field of the Invention
The present invention relates to a communication system, and more particularly to a power amplification apparatus in a communication system.
2. Description of the Related Art
Currently, in a mobile communication system, power amplifiers are disposed at final output parts of base stations (BSs) and mobile stations (MSs), so as to amplify an input signal of a low level, for example, a baseband signal, to an output signal of a high level, for example, a radio frequency (RF) band signal. When a power amplifier is nonlinear, the RF signal has a distorted signal component. Such nonlinearity of the power amplifier reduces the gain of an input signal, and generates a frequency component due to inter-modulation, in addition to the input frequency component. The frequency component generated by inter-modulation exerts an influence on adjacent channels, thereby degrading the entire capacity of the system. Therefore, linearity is a very important factor in the power amplifier.
For this reason, the power amplifier uses a linearization circuit of a feedforward scheme in order to satisfy the linearity. Since the degree of linearization by the linearization circuit is very high, the linearization circuit can satisfy the linearization specification required for a base station amplifier.
FIG. 1 is a block diagram illustrating the operation principle of a conventional power amplifier using a feedforward scheme.
The power amplifier includes a high power amplifier (HPA) 101 for amplifying a low-level input signal to a high-level output signal r, variable attenuators 103 and 109, phase shifters 105, 111 and 115, an error amplifier (i.e. drive amplifier) 113 for detecting and amplifying error of a signal, and delay lines 107 and 117 for delaying an input signal for a predetermined period of time and outputting the signal without any change in the waveform thereof.
The basic operation of the power amplifier using the feedforward scheme is described as follows. First, an original signal S to be amplified is divided into two equal signals S1(t) and S2(t). The signal S1(t) 100 passes through the HPA 101. In order for the HPA 101 to accurately amplify a signal, it is necessary for input and output signals of the HPA 101 to coincide with each other. However, the input and output signals may not coincide with each other due to distortion, noise or the like, generated by the nonlinearity of an element or frequency, so that a noise frequency signal N may be generated. Accordingly, when the signal S1(t) 100 passes through the HPA 101, a noise frequency signal N is generated alongside the original signal S, thereby generating an output signal of S+N.
Meanwhile, the signal S2(t) passes through the variable attenuator 103 and the phase shifter 105. When the signal S2(t) passes through the variable attenuator 103 and the phase shifter 105, the signal S2(t) is converted into signal S′, which has the same power of the signal S2(t) but with a phase inverted by 180 degrees, and then the signal S′ is transferred to the delay line 107.
Also, the composite signal S+N of the signal S1(t) 100 and the noise frequency signal N, is divided into two equal signals 180 and 190, in which signal 180 is transferred to the delay line 117.
The other signal S+N 190 and the signal S′ inverted from the S2(t) meet, so that the signals S and S′ cancel each other with only the noise frequency signal N remains. The noise frequency signal N further passes through the variable attenuator 109 and the phase shifter 111, and is amplified by the drive amplifier 113. The amplified noise frequency signal N passes to the phase shifter 115, in which the phase of the amplified noise frequency signal N is inverted by 180 degrees, thereby generating an inverted noise frequency signal N′.
As a result, the signal S+N 180 passed through the delay line 117 and the inverted noise frequency signal N′ meet, so that the noise frequency signals N and N′ cancel each other, with only the original signal S remaining.
As described above, an amplifier using a linearization circuit of a feedforward scheme requires various modules, such as a variable attenuator, a drive amplifier, a phase shifter, etc., to remove an additionally generated noise frequency signal, thereby degrading the efficiency of the amplifier. Therefore, it is necessary to develop a method to enhance the efficiency of an amplifier.